Polyamines have been shown to be present in many cell types and their concentration within the tissue changes in response to a variety of stimuli. Beta-adrenergic stimulation and hypoxia are associated with an increase in the activity of ornithine decarboxylase, the enzyme responsible for the formation of polyamines, while aging is associated to a decrease in the activity of that enzyme. Additionally, it has been suggested that polyamines may modulate the inotropic state of the myocardium and in particular its response to beta-adrenergic stimulation. We have attempted to define the role of polyamines in cardiac muscle. Isolated rat ventricular myocytes were used to investigate the effect of putrescine spermidine and spermine on the twitch and the associated cytosolic Ca (Cai) transient and, in the unstimulated state on the frequency of spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) and on resting Cai. While putrescine had no effect on contraction and Cai spermidine and spermine had a negative inotropic effect which was due to a decrease in the Cai transient and not to a change in myofilament responsiveness to Cai. In the unstimulated state there was no effect of spermine either on spontaneous SR Ca release or on resting Cai.